It is proposed to continue studies on the mechanisms involved in moving pigment granules in chromatophores of fishes. This is an ideal system in the sense that cells are readily available, they are specialized for pigment motion and their responses to environmental factors can be monitored with the light microscope. Information gathered on these phenomena in these cells can be used to understand similar phenomena in mitosis and axoplasmic transport. Studies to date have indicated that filamentous elements of the cytoplasmic ground substance move the pigment through the aggregate effect of shortening and/or elongating. Of these two activities, the elongation is ATP dependent. Antibodies prepared against tubulin have helped at the light microscope level to observe the organization of the cytoplast as it relates to microtubules. This is particularly striking in cells belonging to the epidermis overlying the chromatophores. That microtubules are essential to the directed motion of pigment has been demonstrated by partially disassembling the microtubules with colchicine and nocodazole. In their absence the motility mechanism cannot function. It is important now to examine the mechanism for the presence of actin and myosin and their distribution relative to the microtubules. This will be done at both the light and electron microscope levels with labeled antibodies. Finally, we shall pursue the usefulness of tannic acid as a mordant for the staining of centrosphere elements which function in microtubule assembly. The aim here is to digitize in serial sections the location of these elements for subsequent computer-guided reconstruction in three dimensions.